Calcitonin (CT) was originally identified as an endocrine hormone that lowers extracellular calcium by inhibiting osteoclastic bone resorption and enhancing renal calcium excretion. The identification of CT receptors in diverse tissues and cell types and the demonstration of its effects on the developing embryo indicates that CT may serve a more complex and diverse function than its role in regulating homeostasis. The discovery that the calcitonin receptor (CTR) gene is alternatively spliced and that certain CTR isoforms may bind ligands other than CTL provides further insight into the molecular and biochemical basis for this functional diversity. In the proposed studies, the Principal Investigators will 1) Clone and characterize the murine CTR gene and identify the transcription start site(s) in osteoclasts and other mCTR-expressing cell-types. 2) Define the specific and potentially unique regulatory sequences responsible for expression of mCTR expressing cells and tissues and identify the putative transacting transcription factors that bind to these sequences. A variety of experimental approaches will be used to define the factors controlling mCTR expression in osteoclasts and other cell types during development. This will include defining the sequences involved in the regulation of mCTR gene expression by calcitonin (CT) and other hormones (e.g., dexamethasone) or cytokines (e.g., IL-1). It is anticipated that the regulatory factors that control mCTR gene expression in osteoclasts also are involved in the regulation of the expression of other genes that accompany the final stages of osteoclast differentiation and activation. 3) Investigate the developmental regulation of the mCTR gene. The temporal and cell-specific pattern of mCTR gene expression will be documented, and then transgenic mCTR-lacZ reporter mice will be used to analyzed the relevance of the regulatory regions identified in Aim 2 and to define any other sequences that may be important for developmentally correct expression in vivo. 4) Determine the effect of targeted disruption of the mCTR gene on osteoclasts and other mCTR-expression tissues, and on mouse development. The Principal Investigator and her colleagues and her colleagues will generate transgenic mice with a targeted disruption of the mCTR gene. In addition, they will develop ES cell differentiation techniques to generate osteoclasts from E cells for studying the regulation of mCTR gene expression during osteoclast differentiation and development.